Affiliate Marketing Secrets

I have created two forms sale and purchase .in purchase form I have two text boxes cost per item1 and cost per item2. I need to store these inputs cost per item1 and cost per item2 in ms access table then when I use sale form to enter sale data’s (I have two text boxes quantity sold1 and quantity sold2) and click on save button, I want to get the total cost of both unit sold such as (cost per item1*unit sold1) + (cost per item2*unit sold2)
I can do it using Vb.net and excel but I don’t know how to deal with it in ms access. Any bit of info would be helpful.